Morphological instabilities, or structural instabilities, of thin films on a substrate block the production of numerous technology building bricks. This destabilization occurs during technology steps such as annealing, or if epitaxial stresses or mechanical stresses are present, such as those that result from temperature variations in the case of materials with different coefficients of expansion. The degree of destabilization is proportional to the difference between the surface energies of the film and the substrate. It can be amplified or accelerated by the existence of surface defects, defects at the interfaces or bulk defects, and is inversely proportional to the thickness of the film. In SOI devices, it is typical nowadays to seek to obtain films with a thickness of less than 10 nm, in which problems of the above type are liable to occur.
Dewetting and spreading are significant consequences of morphological instabilities. In the technical field to which the invention relates, solid phase dewetting is defined as a reduction in the area initially occupied by the layer on the substrate. In contrast, spreading is defined as an increase in the area. In certain particular cases, instability does not change the area between the layer and the substrate and modifies only the envelope of the layer, as represented in FIG. 1.
The hypotheses traditionally invoked to explain structural instabilities of monocrystalline thin layers are surface diffusion of material (H1), surface (and interface) energy, which depends on the crystal orientation concerned (H2), mechanical stress resulting from processes (whether suffered or applied intentionally) (H3), and chemical decomposition of the layer in the case of alloys (H4). In the case of polycrystalline thin layers, the influence of the grain boundaries is another destabilizing factor that has to be considered.
The routine solution to alleviate morphological instabilities is to encapsulate the thin layer in a material that controls the phenomena occurring in H1, H2 and H3. For example, if an SOT layer retains a thin surface layer of thermal or native Si oxide (of the order of 1.5 nm thick), the Si film becomes very stable even at high temperature (above 850° C.): surface diffusion of Si is blocked (H1), the surface energy of SiO2 is lower than that of Si, and the elastic constraint remains low even for large variations in temperature. However, in several cases, such as epitaxial regrowth, the use of encapsulation is incompatible with the remainder of the process. Moreover, even if this layer is removed, it can then cause residual chemical contamination of the initial thin layer and modify the expected properties of the material or introduce other instabilities.
Using a low surface tension surfactant (for example antimony for a silicon film or gallium or bismuth for a gallium nitride film) can also be envisaged for alleviating dewetting linked to the surface energy difference (H2) and/or to modify surface diffusion (H1). However, this solution has the same drawbacks (cleaning difficulties).
The document “Reactive Solid State Dewetting: Interfacial Cavitation in the System Ag—Ni—O”, H. de Monestrol, L. Schmirgeld-Mignot, S. Poissonnet, C. Lebourgeois, and G. Martin, Interface Science 11 (1003) 379-390 discloses that, the surface being an infinite source of vacancies, there exists between the surface and the volume a continuous stream of vacancies that can induce condensation of vacancies in the vicinity of the interface (I) of the layer (C) and the substrate (S), as shown in FIG. 2 for several layer thicknesses. Beyond a certain concentration, the vacancies join up to form cavities that become sources of instability that can lead to dewetting of the layer; moreover, this effect becomes stronger as the thickness of the layer is reduced, as can be seen clearly in FIG. 2.
Moreover, it is known that these mechanisms are exacerbated in particular by the presence of surface structural defects (for example, pinholes, emergent dislocations, roughness, and the like) or bulk structural defects (grain boundaries), and by the presence of defects of a chemical nature (departure from stoichiometric conditions, presence of foreign bodies and segregation, and the like) or by the conditions at the boundaries of the thin layers (edge effect, geometry effect, size and orientation).
A method of reducing certain defects in crystals of silicon on sapphire (SOS) by amorphization by implantation of silicon ions followed by annealing is disclosed, for example, in the papers “Reduction in crystallographic surface defects and strain in 0.2-μm-thick silicon-on-sapphire films by repetitive implantation and solid-phase epitaxy”, Golecki I. et al., in Applied Physics Letters, AIP, American Institute of Physics, Melville, N.Y., US, Vol. 40, No 8, April 1982 (1982-04) and “Improvement of crystalline quality of epitaxial silicon-on-sapphire by ion implantation and furnace regrowth”, Golecki I. et al., Solid State Electronics, Elsevier Science Publishers, Barking, GB, Vol. 23, NO 7, July 1980.
PCT patent application no. WO 2004/095 553 describes the preparation of structures including strained films on a substrate. The basic principle relies on the use of ion implantation to produce a defect area within a stack of layers (in/under certain of them) and thus enable relaxation of certain of them prior to heat treatment.
The paper “Crystalline quality improvement of SOS films by SI implantation and subsequent annealing”, Inoue T. et al., in Nuclear Instruments & Methods in Physics Research, North-Holland Publishing Company. Amsterdam, NL, Vol. 182/183, no. Part 2, April 1981, describes a method of improving the quality of SOS films by Si ion implantation and amorphization.
The paper “Electrical and crystallographic evaluation of SOS implanted with silicon and/or oxygen”, Yamamoto Y. et al., in Nuclear Instruments & Methods in Physics Research, section-B: Beam interactions with materials and atoms, Elsevier, Amsterdam, NL, Vol. B7/8, n. 1, Part 1, A March 1985, concerns a process that relates in particular to SOS type devices. The objective of the process is to improve the quality of the crystals by inhibiting diffusion of aluminum atoms from the substrate to the silicon layer. For this the authors recommend oxygen ion implantation or co-implantation of ions of oxygen and ions of silicon so that the silicon is amorphized.
Finally, U.S. Pat. No. 4,617,066 proposes a method of preparing crystals having abrupt P and N regions. The method necessitates an amorphization step after which ions are implanted in the structure.
However, no solution to the problem of morphological instabilities envisages the influence of volume diffusion, in particular the role of vacancy diffusion, and current techniques remain somewhat unsatisfactory as to their response to the drawbacks of one of the more serious phenomena in this field.